Addition agents in manufacture of steel



United States Patent 2,785,970 ADDITION AGENTS 1N MANUFACTURE OF STEELEdward A. Loria, Niagara Falls, N. Y., assignor to The CarhorundumCompany, Niagara Falls, N. Y., a corporation of Delaware No Drawing.Application December 3, 1953, Serial No. 396,056

5 Claims. (Cl. 75-58) This invention relates to the production of steel,and in particular to the production of deoxidized wrought steels andsteel castings substantially free from pinhole porosity.

, Heretofore, in the production of deoxidized wrought steels, and steelcastings many dilferent addition agents have been employed to removeoxides and oxygen from the steel and to prevent pinhole porosityin steelcastings. For example, :as .a final deoxidant in steel-making suchmaterials as aluminum, calcium-silicon, and silicon carbide have beenused, whereas to prevent pinhole porosity in steel castings it has beenthe practice to add to the steel aluminum, either alone or incombination with another deoxidant such as calcium silicide.

While the heretofore used deoxidants are satisfactory for some types ofsteel, they have not been totally satisfactory under certaincircumstances. If only aluminum is added to steel of low carbon contentin sufli-cient quantity to prevent pinhole porosity in steel castingsthe resulting productsmay possess an undesirable inclusion structure. Onthe other hand, if aluminum is added in conjunction with anotherdeoxidant the grain structure of the castings is not completelysatisfactory for all uses.

Similarly, in the production of low carbon killed steel ingots, the useof either silicon carbide or calcium-silicon alone does not always givethe required sound ingots. On the other hand, the use of aluminum aloneresults in sound ingots, but the initial wrought products made from suchingots usually contain inclusion segregates or stringers thatundesirably efiect tensile properties and ofte interfere with furtherfabrication processes.

Because of this inability to obtain completely satisfactory non-metallicinclusion type and distribution when using a single deoxidant, it hasbecome common practice to use a combination of several chemical elementsfor deoxidizin-g low carbon killed steel. While some of these deoxidantcombinations give a product satisfactory for many uses, none of thecommonly used deoxi-dants or combination of deoxidants give a productwith inclusion and grain structures which are completely satisfactoryfor i all uses.

it is therefore an object of the present invention to pro vide a processfor producing deoxidized steels. Another object is to provide a processfor making steel castings which are substantially free from pinholeporosity. A

further object is to provide a method for producing deoxidized steelsand steel castings having superior inclusion and grain structures.

Other objects and advantages accruing from the practice of the presentinvention will become apparent as the description proceeds. g

It has been found that the above and other objects may 2,785,970Patented Mar. 19, 1957 "ice ' principles of this process, as applied tothe making of steel castings, are substantially applicable to theproduction of deoxidized ingot steel.

In the making of sound steel castings it is necessary to add to themolten steel a substance which will prevent '-the formation of numeroussmall voids in the castings,

which voids are commonly referred to collectively as pin hole porosity.In the past, to prevent pinhole porosity, it has been the practice toadd aluminum to molten steel which is tobe used for castings, eitheralone or in conjunction with calcium-silicon. There have been numerousattempts to prevent pinhole porosity by using deoxidants such ascalcium-silicon and silicon carbide without also using aluminum, butthese attempts have met with very little success. Prior to the presentinvention, to prevent pinhole porosity it has always been thought to benecessary to employ aluminum as an addition agent, despite the fact thatthe resulting castings can and usually do contain inclusion formationsdeleterious to tensile andductility properties.

lt has now been discovered that by using aluminum carbide as theaddition agent to prevent pinhole porosity in steel castings, perfectlysound castings can be produced, \which castings have certain superiormechanical properties. It has also been found that by using, as anaddition agent to molten steel, aluminum carbide in conjunction beaccomplished by using aluminum carbide, either alone or. in conjunctionwith one or more other deoxidants, as an addition agent in theproduction of steel. When introduced' into the molten steel thealuminumcarbide decomposes into aluminum and carbon in the nascentstates,

with at least one material selected from the group con sisting of.aluminum, silicon carbide, and calcium-silicon, steel castings can bemade from this steel which have no trace of pinhole porosity and whichhave highly desirable grain and inclusion structures.

The following examples show some of the applications of the processes ofthe present invention.

' EXAMPLE I To prevent pinhole porosity in steel castings made from lowcarbon steel, additions of impure aluminum carbide in combustiblecontainers amounting to 6 pounds per ton of molten steel were added tothe shank ladle before the molten undeoxidizedmetal was drawn from thebull or receiving ladle into the shank ladle. Immediately after themolten steel hadbeen drawn into the shank ladle onto thealuminum-carbide, steel castings were poured, which castings werespecifically designed to .reveal and accentuate pinhole porosityformation. Examination of the resulting castings showed them to besound, there being no traces of pinhole-porosity.- The impure aluminum,carbide used in this operation was made in an electric-- resistancefurnace, chemical analysis:

Percent f Total aluminum} including that presentas aluminum rb dThefaddition rs pounds of this g radealumimnmcar f bide was sufficientto completely prevent pinhole porosity inthesecg stinss.

It had the following composition by 7 of mix sufliciently,

3 EXAMPLE 11 Procedure similar to that in Example I was followed,

but with only 4 pounds per ton of the same grade of aluminum carbidebeing added to the molten.steel. The.

resulting castingsrshowed no trace of pinhole porosity. The addition ofonly 4 pounds of this grade aluminum carbide was sufficient tocompletely prevent pinhole porosity in these castings.

EXAMPLE III Procedure similar to that in the above examples wasfollowed, making 4 and 6 pounds per ton additions of electric resistancefurnace aluminum carbide having the following composition by chemicalanalysis:

Total aluminum including that present as aluminum carbide.

An examination of the resulting castings showed that those made from thesteel which had been treated with the 6 pounds per ton additions werecompletely free from pinhole porosity. The castings made from steel towhich was added only 4 pounds per ton aluminum carbide were slightlyporous, but were suliiciently sound for many uses. The addition of 6pounds of this grade aluminum carbide was sufiicient to completelyprevent pinhole porosity in these castings, and the addition of 4 poundsof this grade aluminum carbide was sufiicient to prevent pinholeporosity to such an extent to produce castings satisfactory for useunder less drastic conditions.

EXAMPLE IV Procedure similar to that in the above examples was followed,but adding 2 pounds of aluminum carbide of the grade used in Example Iplus 2 pounds of calciumsilicon. The resulting castings were perfectlysound, there being no trace of pinhole porosity.

EXAMPLE V Procedure similar to that in the above examples was followed,but adding 2 pounds of aluminum carbide of the grade used in Example IIIplus 2 pounds of aluminum shot. The resulting castings were perfectlysound, there being no trace of pinhole porosity.

By following a procedure similar to that in the above examples, pinholeporosity can be prevented in steel castings by adding aluminum carbideplus silicon carbide.

Aluminum carbide of varying purity can be" made in electric resistancefurnaces by passing an electric current through a carbon core to heat asurrounding mixture of pure aluminum or some other source of aluminumsuch as alumina and a source of carbon such as graphite in thestoichiometric proportions for forming aluminum carbide. The furnaceproduct can be used directly in most instances, no further purificationbeing required. The resulting material adjacent the carbon core is ofhigher purity than that in the outer layers, and is the most desirableas an addition agent. If the furnacing operation is not continued. for along enough time to heat the outermost layers the reaction in theselayers will not progress to any great extent, in which case it may beundesirable. to use this outermost material asan addition agent. If so,this. material can be used as part of the raw mix for subsequent.furnace runs.

Instead of adding the aluminum carbide, with or without othersubstances, to the shank ladle, as was done in the-above-examples, theaddition can-be made to'the' steel while it is stillv in the bull ladle.Also the addition can be made to the bull ladle prior to or duringtapping of the '6 pounds per ton for the best results.

steel into the bull ladle. This procedure gives thorough mixing of thealuminum carbide with the molten steel whereby the aluminum carbide isdecomposed, resulting in satisfactory deoxidation of the steel. Toprevent pinhole porosity in the resulting castings, aluminum carbidesuch as that used in the above examples, when used without otherdeoxidants, should be added in amounts from 4 to If less pure aluminumcarbide is used, as much as 8' pounds per ton may be required to producesatisfactory castings. In certain casting designs which are not prone topinhole formationand in steel compositions which do not require strongdeoxidation, less than 4 pounds of aluminum carbide per ton of steel maybesufiicientf To determine whatetfect the present invention of usingaluminum carbide, either alone orin conjunction with another substance,as a final deoxidant has on the tensile properties of the low carboncasting steel, test bars were cast from steel which had been treatedwith 4 pounds per ton and 6 pounds of aluminum carbide per ton, 2 poundsof aluminum carbide plus 2 pounds of metallic aluminum per ton, and 2'pounds of aluminum carbide and 2 pounds calcium silicide per ton. Thealuminum carbide used was the less pure grade such as used in ExampleIII. These test bars were normalized at 1800 F. for three hours attemperature and then tempered at 0 F. for three hours at temperature.Table I shows the tensile properties of the test bars made from thesesteels and also the average tensile properties of bars cast from thesame steel deoxidized in the customary manner, namely with 3 pounds ofcalcium-silicon and 2 pounds of aluminum, and heat treated asabove-described.

' The minimum value specifications for this type of cast alone or inconjunction with another deoxidant, have an averageyield pointapproximately 14% higher than that of thost'eeltreated with 3 pounds ofcalcium-silicon plus 2 pounds of aluminum.

To study the effect of the practice of the present invention on thegrain size of the steel castings, picral etched photomicrographs weretaken of the grain structures of test coupons cast from steels treatedwith various amounts of aluminum carbide, both alone and in conjunctionwith: other substances. A study of these photomicrographs showed thesteel castings made in accordance with the presentinventionto have Wellrefined grain struc tures. A comparison of these photomicrographs withphotomicrographs of the grain structures-of castings made from. steel.treated: in accordance with the commonly followed prior art methodsshowed that aluminum carbide is just as potent, if not more so, on apound for pound basis as metallic aluminum in grain-refining power.

To determine the nature of the inclusion structures ofunetcliedphotomicrographs were taken of test coupons cast from steelstreated with various amounts of aluminum carbide, both alone and inconjunction with other deoxidants. These photomicrographs showed thatsteel castings made in accordance with the present invention have a mostdesirable inclusion structure. Those castings to which the largeradditions of aluminum carbide were made contained substantially none ofthe undesirable types of inclusions which are usually present incastings deoxidized with aluminum.

While the present invention has been described as it pertains to makingsteel castings, as aforementioned, aluminum carbide, either alone or incombination with other deoaidants, is a highly satisfactory finaldeoXider for ingot steels. The additions can be made in the ladle priorto filling it with molten steel, or the additions can be made as theladle is filled. The aluminum carbide may be shoveled into the ladle inloose granular form or it may be introduced into the ladle incombustible or metal containers, or in the form of bonded briquettes.For satisfactory deoxidation of killed-ingot steels about 2-6 pounds perton of aluminum carbide of the purity of those in the examples, whenused alone, should be used. The larger amounts are required for lowercarbon steels (below 0.30% C.) and the smaller amounts are required forhigher carbon steel (above 0.30% C.). If other deoxidants are used incombination with the aluminum carbide, proportionately lesser amounts ofaluminum carbide are required so that only about one pound per ton maybe required. Also, as the purity of the aluminum carbide is lowered theamounts required will be proportionately increased so that up to about 8pounds per ton may be required in killing steel with a high oxidationlevel. Aluminum carbide is also highly satisfactory as a final deoxidantladle addition for semi-killed and rimmed steels, with lesser amountsusually being required to deoxidize these types of steel to the desiredlevel.

Where the term-deoxidized steel is used in the specification and claimsit is intended to include not only ingot steel which has been deoxidizedby an addition agent, but also cast steel which has been treated with anaddition agent to reduce or eliminate pinhole porosity in the resultingcasting, since it is believed that addition agents reduce or eliminatepinhole porosity by means of a strong deoxidizing eifect on the steel.

Having described the invention, it is desired to claim:

1. In the process of producing deoxidized steel the step of introducingaluminum carbide into the molten steel in proportion of from about 1 to8 pounds of aluminum carbide per ton of steel whereby the aluminumcarbide is decomposed and the steel is deoxidized.

2. In the process of producing deoxidized steel the step of introducingaluminum carbide into molten steel in a ladle in proportion of fromabout 2 to 6 pounds of aluminum carbide per ton of steel whereby thealuminum carbide is decomposed and the steel is deoxidized.

3. In the process of producing steel castings substantially free frompinhole porosity the step of introducing into molten steel in a ladlealuminum carbide in proportion of from about 2 to 6 pounds of aluminumcarbide per ton of steel.

4. In the process of producing steel castings substantially free frompinhole porosity the step of introducing into molten steel in a ladle inproportion of from about 4 to 8 pounds per ton of steel a mixturecomprising aluminum carbide and at least one material selected from thegroup consisting of aluminum, silicon carbide, and calcium-silicon.

5. In the process of producing deoxidized steel the step of introducinginto the molten steel in proportion of from about 4 to 8 pounds per tonof steel a mixture comprising aluminum carbide and at least one materialselected from the group consisting of aluminum, silicon carbide, andcalcium-silicon.

References Cited in the file of this patent UNITED STATES PATENTS1,732,915 Saklatwalla Oct. 22, 1929 FOREIGN PATENTS 499,647 GreatBritain Jan. 23, 1939

1. IN THE PROCESS OF PRODUCING DEOXIDIZED STEEL THE STEP OF INTRODUCINGALUMINUM CARBIDE INTO THE MOLTEN STEEL IN PROPORTION OF FROM ABOUT 1 TO8 POUNDS OF ALUMINUM CARBIDE PER TON OF STEEL WHEREBY THE ALUMINUMCARBIDE IS DECOMPOSED AND THE STEEL IS DEOXIDIZED.